Glutathione S-transferases (GSTs) are involved in many stress responses in plants, for example, participating in the detoxification of xenobiotics and limiting oxidative damage. Studies examining the regulation of this gene family in diverse plant species have focused primarily on RNA expression. A proteomics method was developed to identify GSTs expressed in Arabidopsis seedlings and to determine how the abundance of these proteins changed in response to copper, a promoter of oxidative stress, and benoxacor, a herbicide safener. Eight GSTs were identified in seedlings grown under control conditions, and only one, AtGSTU19, was induced by benoxacor. In contrast, four GSTs, AtGSTF2, AtGSTF6, AtGSTF7, and AtGSTU19, were significantly more abundant in copper-treated seedlings. The different responses to these treatments may reflect the potential for copper to affect many more aspects of plant growth and physiology compared with a herbicide safener. Differences between RNA and protein expression of GSTs indicate that both transcriptional and translational mechanisms are involved in regulation of GSTs under these conditions.